The invention relates to a method of manufacturing a magnet from a magnetic material the main phase of which comprises an intermetallic compound of the rare earth transition metal type which also includes boron, comprising the steps of:
(a) forming an alloy of the constituent rare earth and transition metals with the addition of boron,
(b) comminuting the alloy to an average particle size in the range 0.3 to 80 .mu.m and preferably less than about 10 .mu.m,
(c) forming a magnet body by pressing the resulting powder in a pressing tool while the powder is situated in a magnetic aligning field,
(d) sintering the magnet body at a temperature in the range of about 800 to 1200 degrees C., followed by slow cooling, and
(e) after, if necessary, machining to shape, magnetising the magnet body.
The manufacture of such magnets is described in European Patent Application No. 101552.
Magnetic materials based on intermetallic compounds of certain rare earth metals with transition metals may be formed into permanent magnets having coercive fields of considerable magnitude, namely of several hundred kA/m. One method of manufacture includes alloying the constituent materials in an inert atmosphere or in vacuo. The alloy is then comminuted into particles whose average size lies in the range 0.3 to 80 .mu.m and is preferably less than about 10 .mu.m, which are aligned in a magnetic field while being formed into a magnet body by compacting under a pressure of about 10 kN/cm.sup.2. The alignment of the particles is fixed and the particles are bonded together by sintering in an inert atmosphere or in vacuo at a temperature in the range of approximately 800 to 1200 degrees C.
Initially, samarium cobalt (SmCo5) magnets were produced, but they were expensive owing to the scarcity of samarium. Recently, however, new types of rare earth transition metal magnets have been devised using the more abundant rare earth metal neodymium in combination with iron and a small proportion of boron. A typical alloy contains a major hard magnetic phase as Nd2Fe14B, and is of the form Nd15Fe77B8. Although such magnet alloys can have slightly varying compositions they will be referred to herein generally by Nd--Fe--B. One form of Nd--Fe--B magnet has been manufactured with a coercivity of approximately 80 kA/m (10 kOe) and an energy product (B.H) of approximately 240 kJ/m3 (30 MGs.Oe).
It should be noted, however, that other rare earth metals, such as for example, praseodymium or dysprosium, which are less abundant than neodymium and niobium, are sometimes substituted for part of the Nd content of such alloys, as is cobalt for part of the iron content. However the designation Nd--Fe--B will be used herein generally to refer to commercially useful neodymium ion boron magnet alloys whether partially substituted or not.
In one method, the manufacture of an Nd--Fe--B magnet commences with the formation of the bulk alloy suitably by induction melting followed by casting, and the resultant bulk ingot is then broken up and comminuted to a fine powder. Initially comminution was effected by firstly stamp milling to a coarse powder of, for example, 35-mesh sieve followed by fine pulverisation in a ball mill for about 3 hours to the required size of, for example, 3 to 10 .mu.m. This process is slow and cumbersome and it has recently been proposed by I.R. Harris et al in the Journal of Less Common Metals 106 (1985), L1 that fairly large pieces of alloy of about 1 to 2 cm3 can be rapidly broken down into a relatively fine powder of particle size less than 1 mm by hydrogen decrepitation using pure hydrogen at room temperature. This can be carried out in a stainless steel hydrogenation vessel and takes the form of an exothermic reaction resulting in the formation of hydrides of the alloy phases. The resultant powder is then further reduced in size by milling in an attritor mill under cyclohexane for about 25 minutes, as described by P. J. McGuiness et al, J of Materials Science 21 (1986), 4107-4110. Alternatively the resultant powder can be jet milled using nitrogen as a propellant.
This manufacturing process suffers certain disadvantages in that hydrogen gas presents a high degree of explosion risk necessitating elaborate industrial precautions. Furthermore the use of cyclohexane in the attritor mill also constitutes a serious fire risk.